The present invention relates generally to implantable medical interventional devices, and more particularly to improvements in techniques and devices for determining the capability of an implanted cardiac defibrillator to defibrillate the implant patient and to do so with an adequate safety margin.
Considerable progress has been made in recent years in the field of implantable defibrillators, primarily in reduction of device size, weight, and volume attributable to improved electronics, a better understanding of and reduction in defibrillation threshold (DFT), and the consequent use of smaller batteries and capacitors. A major clinical benefit of the reduction in physical characteristics of the defibrillator is that it may be implanted in the pectoral region, in contrast with lower abdominal areas for the bulkier prior art defibrillators. This factor alone has contributed to a marked reduction in the complexity of and time required to perform the implant procedure. Reduced morbidity and fewer complications associated with the pectoral implant procedure has been demonstrated in studies by various practitioners (see, e.g., E. Hoffmann et al., "Chronic Experience with Pectoral Defibrillator Implant," Pace, 18: 888A372 (1995)).
Additionally, the use of the so-called "active can configuration," in which the metal housing or case of the device (often referred to as the "can") enclosing the electronics and batteries is used as one of the poles for administering the defibrillating shock, has led to further simplification of the implant procedure for defibrillators. The second pole of the configuration is an endocardial defibrillation electrode or coil on the implanted lead, which is situated in the cavum of the patient's right ventricle when the lead is fully implanted. The combination of reduced size, weight, and volume, along with pectoral implant site, active can configuration, and other improvements have enabled the defibrillator to be implanted in a procedure ranging from sixty to ninety minutes (see, e.g., H. Schmidinger et al., "Learning Effect in Defibrillator Implant Duration," Pace, 18: 885A359 (1995)).
One of the applicants herein has developed and performed an implant procedure which is even faster and less risky, with the patient subjected only to a local anesthetic and sedation. In this procedure, it is not necessary to administer a general anesthesia intubation narcosis, which represents a considerable advantage for patients with reduced myocardial function, characteristic of the majority of candidates for an implantable defibrillator. Consequently, a major risk of further reducing an already lowered cardiac output in patients who are prime candidates for the procedure, attributable to inhalative narcotics used in general anesthesia, is avoided. Use of local anesthesia and a short action sedation such as versed or propofol for the time during which the shock is applied has further reduced morbidity and mortality of the implant procedure.
Despite these improvements, the technique typically used to test the capability of the implanted device to defibrillate is a debilitating and all too often deadly procedure for the patient. At least two and often three or more DFT determinations are made--by repeatedly inducing ventricular fibrillation (VF) through such means as fast ventricular stimulation, or by applying 50 or 60 Hertz (Hz) alternating current or a T-wave shock, after which the VF is sought to be terminated by a defibrillating shock is delivered to the ventricular mass by the implanted device. The predetermined energy level of the shock is calculated to provide a safety margin of about 10 joules, which is usually deemed sufficient to assure termination of the VF in even the worst cases, namely, where a high ventricular DFT is present and/or where the VF is a certain type which is very difficult to terminate. Nevertheless, the uncertain nature of fibrillation makes the repeated inducement and termination of VF an important confidence builder for the implanting physician, that the implanted device will consistently provide a safe and effective therapy to defibrillate the patient's heart.
Testing by repetitive inducement of VF is used not only during the initial implant procedure, but also in pre-hospital discharge testing and in patient follow-up visits to the physician. Shocks of adequate energy content, field strength, and vectoral component will usually defibrillate. During the procedure, the intracardiac or surface EKG signal is monitored by telemetering to a programming console or display terminal, or by strip-chart recording. Although it is important for the implanting physician to be satisfied that the device will operate as intended, the testing procedure has been the greatest risk to patient survival during an implant, notwithstanding that for determining DFT the energy delivered to terminate is typically successively lowered for the repetitions. The concern for survival is greatest for patients with enlarged heart and compromised myocardial function, such as an ejection fraction in a range below 25 percent. Transesophageal echocardiographic measurements, determination of enzymes, measurement of pulmonary wedge pressure, and radionucleide studies have shown that even after one shock, the end diastolic volume of the ventricle increases and the ejection fraction decreases, adding to the risk. Further severe risk is present for patients under antiarrhythmic drug treatment at times when VF is being induced in connection with the implant procedure. Further reduction of an often compromised myocardial function is an outcome, which may lead to a critical drop in blood pressure, prolonged operating time, and unwanted increase in intrinsic catecholamine stimulation.
Accordingly, it is a principal aim of the present invention to provide an improved method and technique for testing the capability and efficacy of a defibrillator which is in process of being implanted, or which already has been implanted in the patient, to terminate an episode of ventricular fibrillation, and to do so in a way that offers markedly reduced risk to the physical well-being of the patient.